The invention relates to a process for the vacuum coating of an extruded material, particularly an animal food, in which the extruded material is coated with a flowable coating material during a vacuum phase and under reduced pressure.
For manufacturing and product-related reasons, in the case of numerous extruded products, particularly in the human and animal food sector, the need exists not to have certain constituents of the of the finished and product, e.g. fatty substances in the mixture to be extruded, but to add the same only following the extrusion process, so that an extruded product serves as a carrier for the substance to be subsequently applied.
A known procedure consists of providing a product, which can be porous after extrusion, with desired constituents. The extrusion product is initially dried and then exposed to a vacuum and is coated under reduced pressure with a coating material, which has a flowable consistency. At the end of the vacuum phase the increasing pressure forces the coating material into any existing pores of the extruded material, so that the latter has an increased content of e.g., fat, without the latter being preponderantly located on the surface and leading to an excessive stickiness of the product. The porous, extruded product consequently in part mainly serves as a carrier for the flowable material to be added. In this way, at a vacuum of e.g., 200 mbar, fat contents of up to 30 wt. % are obtained in the end product.
It is a disadvantage of this proven coating process that the total costs for drying and coating are high and also the absorptivity of the extruded product for a coating material is not always adequate, even when using vacuum.
The problem of the present invention is to improve the known coating process in this connection, i.e., in particular to lower the drying costs and also bring about increased productivity for the coating material.
According to the invention, this problem is solved by a process for the vacuum coating of an extruded material, particularly animal food, in which the extruded material is coated with a flowable coating material during a vacuum phase and under reduced pressure. The extruded material, during the vacuum phase, is initially dried and then coated with the coating material. The drying of the extruded material during, instead of before, the vacuum phase surprisingly leads to several advantages. Firstly, the already porous, extruded material expands further as a result of the evaporation of water during the vacuum phase, so that pores are produced or the pore volume is increased and a correspondingly increased absorptivity for flowable coating material is obtained. In addition, in the case of a vacuum, the drying process can be much better controlled than e.g., drying in hot air, because under reduced pressure there is an evaporation of the inwardly located material areas, which would otherwise not be accessible to contact with drying air. As a result smaller local fluctuations of the moisture content of the product occur, so that the mean moisture content can be raised approximately 1%, which is obviously associated with a considerable cost saving. As a result of the extraction of heat in the drying process the extruded product is simultaneously cooled, so that there is no need for a separate cooling of the material leaving the extruder at a temperature of approximately 100xc2x0 C.
The coating material can contain fat and/or water.
Preferably, the extruded material is porous and expands during the vacuum phase.
Preferably, at the start of the vacuum phase the extruded material has a temperature higher than 90xc2x0 C.
According to a preferred embodiment, at the start of the vacuum phase, the extruded material has a moisture content, related to its dry, weight, of no more than 25 wt. %.
According to an embodiment of the invention, during the vacuum phase, the moisture content of the extruded material is lowered by 6 wt. %, based on the dry weight.
Prior to the start of the vacuum phase, the extruded material can be predried at normal pressure.
Preferably, after drying and coating, the extruded material has a final moisture content of 9 wt. %, based on its dry weight. After drying and coating, the extruded material preferably has a temperature of 25xc2x0 C.
During the vacuum phase the pressure can be lowered to 200 mbar.
According to an embodiment, at the start of the vacuum phase the pressure is lowered to 40 mbar. The pressure can be kept at 40 mbar until the extruded material reaches a temperature of 30xc2x0 C. or lower.
The vacuum phase can last up to 5 minutes.
During the vacuum phase, it is possible to supply additional energy, particularly in the form of infrared or microwave radiation.